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PHPâs two data types â resource and NULL â are classified as special types. An object of resource type refers to external resources like database connection, file streams etc. On the other hand, a NULL data type is a variable without any data assigned to it. In this chapter, we shall learn more about these types.
Resource Type
A PHP program often needs to interact with an external environment such as a database, or a disk file etc. These are treated as resources in PHP. Resource is a special data type that refers to any such external resource. PHP uses relevant functions to create these resources. For example, fopen() function opens a disk file and its reference is stored in a resource variable.
PHP”s Zend engine uses reference counting system. Hence, a resource with zero reference count is destroyed automatically by garbage collector and the memory used by resource data type need not be freed manually.
Different built-in PHP functions return respective resource variables. Subsequently, PHP uses them for interacting with the corresponding external environment. For example, the fopen() function returns a file resource, which acts as a file handle and the read/write operations on the file are facilitated by this resource variable.
The following table summarizes different functions that return resource variables −
| Resource Type | Built-in functions | Definition | |
|---|---|---|---|
| Produced | Sold | ||
| bzip2 | bzopen() | bzclose() | Bzip2 file |
| curl | curl_init() | curl_close() | Curl session |
| ftp | ftp_connect(), | ftp_close() | FTP stream |
| mssql link | mssql_connect() | mssql_close() | Link to Microsoft SQL Server database |
| mysql link | mysql_connect() | mysql_close() | Link to MySQL database |
| mysql result | mysql_db_query(), | mysql_free_result() | MySQL result |
| oci8 connection | oci_connect() | oci_close() | Connection to Oracle Database |
| ODBC link | odbc_connect() | odbc_close() | Link to ODBC database |
| pdf document | pdf_new() | pdf_close() | PDF document |
| stream | opendir() | closedir() | Dir handle |
| stream | fopen(), tmpfile() | fclose() | File handle |
| socket | socket_create() | Socket_close() | Socket handle |
| xml | xml_parser_create() | xml_parser_free() | XML parser |
| zlib | gzopen() | gzclose() | gz-compressed file |
| zlib.deflate | deflate_init() | None() | incremental deflate context |
| zlib.inflate | inflate_init() | None() | incremental inflate context |
PHP has get_resource_type() function that returns resource type of a variable.
get_resource_type ( resource $handle ) : string
where $handle is the resource variable whose type is to be obtained. This function returns a string corresponding to resource type.
There is also get_resource_id() function an integer identifier for the given resource.
get_resource_id(resource $resource): int
Example
This function provides a type-safe way for generating the integer identifier for a given resource.
<?php
$fp = fopen("hello.php", "r");
$resource = get_resource_type($fp);
$id = get_resource_id($fp);
echo "The resource type is : $resource The resource ID is : $id";
?>
It will produce the following output −
The resource type is : stream The resource ID is : 5
NULL type
In PHP, a variable with no value is said to be of null data type. Such a variable has a value defined as NULL. A variable can be explicitly assigned NULL or its value been set to null by using unset() function.
$var=NULL;
It is possible to cast variable of other type to null, although casting null to other type has been deprecated from PHP 7.2. In earlier versions, casting was done using (unset)$var syntax
Example
The following example shows how to assign NULL to a variable
<?php $var=NULL; var_dump($var); ?>
It will produce the following output −
NULL
Example
The following example performs null variable to other primary variables −
<?php $var = NULL; var_dump( (int) $var); var_dump((float)$var); var_dump((bool) $var) ; var_dump( (boolean) $var); ?>
It will produce the following output −
int(0) float(0) bool(false) bool(false)
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